Gaseous arc discharge device



April 4, 1961 J. M. LAFFERTY 2,978,605

GASEOUS ARC DISCHARGE DEVICE Filed Oct. 17, 1957 //7 ve mor James M.Lofferfy,

His Afforneyl GASEOUS ARC DISCHARGE DEVICE James M. Laiferty,Schenectady, N.Y., assignor to General Electric Company, a corporationof New York Filed Oct. 17, 1957, Ser. No. 690,851

12 Claims. (Cl. 313247) The present invention relates to gaseouselectric arc discharge devices. More particularly, the invention relatesto improved instant-start, high-temperature thyratron devices.

Thyratron gaseous arc discharge devices generally comprise a cathode, ananode and a control electrode or grid. The control electrode is arrangedto provide almost complete shielding between cathode and anode so that asmall negative potential applied to the control electrode prevents agaseous discharge between cathode and anode even when a high potentialis applied therebetween. There is, however, for each value ofanode-cathode potential, a value of control electrode potential whichjust allows the gaseous atmosphere to become ionized. Once this occursan arc discharge is established between cathode and anode and thecontrol electrode loses control. The control electrode becomes effectiveagain only if the anode-cathode potential is decreased to the pointwhere the arc is extinguished. While sustaining an arc discharge,thyratrons exhibit only a few volts potential difference between anodeand cathode. Because of this, thyratrons are quite useful as switchingdevices and the like.

In such applications it is of great importance that the breakdowncharacteristics of the thyratron devices remain constant and stablethroughout the life of the tube. It may also be important that thecharacteristic be independent of temperature and that the device operateat high temperatures.

Accordingly, it is an object of the present invention to providethyratron gaseous arc discharge devices which maintain a stablebreakdown characteristic throughout the device life.

Another object of the invention is to provide thyratron gaseous arcdischarge devices which operate stably over a wide range oftemperatures.

A further object of the invention is to provide instantstart thyratrongaseous arc discharge devices.

Still another object of the invention is to provide thyratron gaseousarc discharge devices having improved heat dissipation characteristics.

In accord with the present invention, I provide thyratron gaseouselectric discharge devices having cylindrical symmetry with concentricanode, control electrode and axial cold arc type cathode. The controlelectrode and cathode are supported by support members which areinsulatingly stacked with the anode cylinder and hermetically sealed toform the device envelope. The interior of the envelope is filled with ahigh purity noble gas. The cathode utilized is. an instant-start arctypecause only metal and ceramic are utilized in its construc- 2,978,605Patented Apr. 4, 1961 vention itself, together with further objects andadvantages thereof, may best be understood by referring to the followingdetailed description taken in connection with the appended drawing inwhich:

Fig. l is a vertical cross-sectional view of a device constructed inaccord with the present invention,

Fig. 2 is a vertical view "of a portion of one type cathode which may beutilized in the device of Fig. 1, and

Fig. 3 illustrates another suitable cathode.

In Fig. 1, a thyratron gaseous are electric discharge device constructedin accord with the present invention comprises a cylindrical metallicanode member -1, a concentric cylindrical mesh control electrode 2concentric within anode 1 and of somewhat greater length, and an axialcold arc type cathode 3 along the axis of the cylinders comprising theanode and control electrode. Control electrode cylinder 2 is supportedat both ends by annular metallic support members 4 which have an insidediameter less than the inside diameter of anode cylinder l andsubstantially the same diameter as control electrode cylinder 2. Cathode3 is supported at either end by a metallic end wall member 5 which hassubstantially the same diameter as the outside diameter of anodecylinder 1. All of the metallic members supporting the cathode andcontrol electrode are separated from one another and from the anodecylinder by a plurality of annular insulating ceramic spacers 6 whichform hermetic seals with the metallic members to enclose the entirestructure and form an hermetically sealed envelope. In operation, theend walls 5 provide a cathode terminal, the exterior of cylinder 1provides an anode terminal, and the support rings 4 each provide controlelectrode terminals. The entire assembly is filled with a highlypurified stable noble gas.

Although anode cylinder 1, control electrode support members 4 andcathode support members 5 may be fabricated of a highly conductivematerial such as copper, they are preferably fabricated of titanium dueto the unique gettering characteristics of titanium. I have found that,in noble gas thyratron gaseous arc discharge devices, thereproducibility of breakdown parameters throughout the life of thedevices is markedly dependent upon the purity of the gaseous atmosphereutilized. Thus, if a substantial quantity of a gaseous impurity such asCO H O, H 0 and N is present within the noble gas atmosphere, thebreakdown characteristics tend to change as the tube ages so that ahigher or lower negative potential is required to inhibit breakdown at agiven anodecathode voltage depending upon the particular impuritypresent. Accordingly, utilizing titanium electrodes, a highly purifiedstable noble gas atmosphere which may comprise any single noble gas suchas helium, argon, neon, krypton, xenon or mixtures thereof, but whichpreferably comprises xenon, may be obtained by mechanically fittingvarious parts of the thyratron device together as shown in Fig. l of thedrawing and sealing the ceramic members to the titanium members in anatmosphere of the noble gas with which the tube is to be charged at asuitable pressure. This process for the formation of noble gas gaseouselectric discharge devices is disclosed and claimed in my copendingapplication Serial No. 690,849, filed concurrently herewith and assignedto the present assignee, now U.S.. Patent No. 2,957,741. V I

When the devices of the present invention are so formed, the titanium isheated to a temperature at which it bonds to the titanium-matchingceramic members 6. At these temperatures (of the order of 700 C. to1100" C.) titanium is an excellent getter for gases such as CO H O, H Nand Thus, While the tube is being formed, the titanium members whichcomprise the tube eifectively remove all impurity gaseous constituentstherefrom leaving a highly purified noble gas, preferably xenon, whichresults in the maintenance of stable breakdown conditions throughout thelife of the device.

In a gaseous arc, as used in devices of the invention, and as opposed toa gaseous glow discharge, the potential difference between anode andcathode is dependent only upon the gas utilized. In most applicationswherein thyratron devices are used, namely in switching applications andthe like, it is of the greatest importance that the potential differenceacross the thyratron be a minimum, leaving the major portion of theavailable potential for the load device to be operated thereby.Accordingly, the devices of the present invention, to great advantage,preferably are filled with an atmosphere of xenon (arc voltage=8 v.),although for certain applications argon (16 v.), neon (2.1 v.), krypton(14 v.) or helium (25 v.) may be used.

Titanium-matching annular sealing members 6 are composed of a refractoryceramic, the coefficient of thermal expansion of which is a close matchfor that of titanium, and which may be suitably bonded at hightemperatures to form hermetic seals with titanium anode cylinder 1,control electrode support members 4 and cathode support disks 5. Such aceramic may be a sintered agglomerate of silicon oxide, magnesium oxide,and aluminum oxide denominated as Forsterite. One such Forsteriteceramic and the method of preparation thereof is disclosed and claimedin the copending application of A. G. Pincus, Serial No. 546,215, filedNovember 10, 1955, and assigned to the assignee of the presentinvention.

Cylindrical mesh control grid 2 may conveniently be formed of stainlesssteel, molybdenum, tungsten or any material which does not reactstrongly with titanium. Preferably the electrode is made from stainlesssteel, since at the temperatures at which the tube may be formed(approximately 700 C. to 1100" C.) stainless steel and titanium form astrong bond with one-another, and no further means is necessary tosecure control electrode cylinder 2 and control electrode supportmembers 4 together. Conveniently cylinder 2 may be approximately 20-20per inch mesh.

Cathode 3 is a cold arc type cathode. As used herein the phrase cold arctype cathode means a cathode having low heat capacity for the volume itoccupies, low thermal conductivity along its length, low spectralemissivity and good thermionic emitting characteristics. When theseconditions are satisfied it is not necessary that the cathode be heatedby an external source, such as by the passage of an electric currenttherethrough, in order to cause the formation of an are between thecathode and an anode. This is because, in a cold arc type cathode, theprocess by which an arc is established is one of the ejection ofsecondary electrons and the bombardment of the cathode by positive ionswhich gradually raise its temperature without external energy beingsupplied therethrough. Thus, when the cathode has a low heat capacity,only a small amount of heat need be added to a particular spot to raiseit to a temperature at which thermionic emission occurs. Similarly, whenthe cathode possesses low thermal conductivity it is possible for onespot upon the cathode to be raised to a temperature at which thermionicemission may occur without dissipating heat rapidly throughout theentire cathode. Likewise, when the cathode possesses low spectralemissivity it is similarly easy to build up the temperature in one spotthereon without the loss of heat energy by radiation. Finally, when thecathode material possesses good thermionic emitting characteristics thecathode temperature at which an arc is established need not beexcessively high.

. One such cathode structure satisfying these conditions comprises adoubly coiled helix of tungsten wire formed into a third loosely woundhelix 8 and coated with a mixed carbonate of barium, calcium andstrontium for the attainment of good thermionic emittingcharacteristics. In Fig. 2 the composite doubly-coiled helix, from whichthe large loosely-wound helix 8 shown in Fig. 1 is formed, isillustrated in detail. No claim is made to the novelty of this cathodestructure per so since it is disclosed and claimed in U.S. Patents Nos.2,306,925 to Aicher and 2,774,918 to Lennners. This cathode, however,possesses the characteristics necessary for an are between cathode andanode in the present device. Thus, since the cathode is formed of atriple coil of fine wire rather than a solid rod, it possesses low heatcapacity. Since the coil is not a single conductor and is fabricatedfrom fine tungsten wire, the electrical conductivity of which isrelatively low, the thermal conductivity of the coil along its length islow. Additionally, heat conduction does not occur along the wire butmust occur through the carbonate coating thereupon which coatingpresents a high impedance to the passage of heat from one coil to anadjacent coil. The carbonate upon the surface of the tungsten wirepossesses good thermionic emitting characteristics and low spectralemissivity. Accordingly, all the characteristics for a cold arc typecathode are satisfied. Cathode leads 7 support cathode 3 in place andmay be heavy rods of the same materials comprising cathode 3.

Other structures may similarly satisfy the necessary criteria for thecathode utilizing the present invention. Thus, for example, asillustrated in Fig. 3, the cathode may comprise a conducting basematerial 10 about which there is woven a finely divided mass of metallicwool fibers 11 coated with a suitable thermionic emitting substance.This cathode likewise, has low heat capacity, poor thermal conductivity,low spectral emissivity and good thermionic emitting characteristics.Such a material, in mass, is disclosed and claimed in my copendingapplication Serial No. 444,939, filed July 22, 1954, and assigned to thepresent assignee.

Since the cathodes utilized in devices of the present invention are coldarc-type cathodes which require no heating, the devices of the presentinvention are instant starting and may be utilized in a number ofswitching applications in which conventional thyratron gaseous arcdischarge devices, which require a finite time for cathodes to becomeincandescent, may not be used.

If the cathodes utilized in the present invention are operated at thegas pressures conventionally utilized in thyratron devices, for example,0.05 to 0.1 mm., the cold arc type cathodes do not possess long-lifecharacteristics. I have found, however, that long life may be obtainedby operating the devices at a pressure of approximately 1 to- 3 mm. ofmercury pressure. For optimum results, this requires that the controlelectrodeanode spacing be reduced because of the reduced mean free pathof ions at this increased pressure. In general, for optimum operation inthe devices of my invention, the product of gas pressure, inmillimeters, and control electrode-anode spacing, in centimeters, shouldbe approximately equal to 2.5 for xenon, 3.5 for helium, 3.5 for neon,1.0 for argon, and 2.25 for krypton.

A further advantage of devices constructed in accord with the presentinvention is derived from the fact that anode cylinder 1 comprises anintegral part of the envelope of the discharge device. Thus, anode 1 mayreadily be liquid cooled or air cooled, facilitating the operation ofthe device at high current densities and at high temperatures, resultingin a greatly increased efliciency. Additionally, since no soft glassparts are utilized in these devices, which comprise only ceramic and a ametal, the devices may be operated at temperatures up to 500 C. with nodeleterious eifects and maintain their operating characteristics stableover this range of tem peratures.

While the invention has been described hereinbefore with respect tocertain embodiments thereof, many changes and modifications willimmediately occur to those skilled in the art without departing from thespirit of the present invention. Accordingly, I intend by the appendedclaims to cover all such modifications and changes as fall within thetrue spirit and scope of the present invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A thyratron gaseous electric discharge device comprising a metallicanode cylinder member; a mesh control electrode cylinder concentric withsaid anode cylinder and interior thereof said control electrode beinglonger than said anode cylinder; a pair of metallic annular supportmembers supporting said control electrode; a cold arc-type cathodeextending axially along the center of said cylinder said cathodeexhibiting characteristics of low heat capacity and low thermalconductivity as compared with metals and adapted to serve as thefootpoint of an arc between anode and cathode in the absence ofresistance heating thereof; a pair of metallic disk-shaped cathodesupport members supporting said cathode and forming end walls for saiddevice; a plurality of insulating ceramic members interposed betweensaid end wall members and said control electrode support members andbetween said control electrode support members and said anode member andhermetically sealed thereto to form an hermetically sealed envelope; anda highly purified stable noble gas atmosphere within said envelope.

2. The device of claim 1 in which said metallic members are of titanium.

3. A thyratron gaseous electric discharge device comprising a metallicanode cylinder member; a mesh control electrode cylinder concentric withsaid anode cylinder and interior thereof said control electrode beinglonger than said anode cylinder; a pair of metallic annular supportmembers supporting said control electrode; a cold arc-type cathodeextending axially along the center of said cylinder; a pair of metallicdisk-shaped cathode support members supporting said cathode and formingend walls for said device; a plurality of insulating ceramic membersinterposed between said end wall members and said control electrodesupport members and between said control electrode support members andsaid anode memher and hermetically sealed thereto to form anhermetically sealed envelope; and a highly purified atmosphere of xenonwithin said envelope.

4. The device of claim 3 in which said metallic members are of titanium.

5. A thyratron gaseous electric discharge device comprising a metallicanode cylinder member; a mesh control electrode cylinder concentric withsaid anode cylinder and interior thereof said electrode being longerthan said anode cylinder; a pair of metallic annular support memberssupporting said control electrode; a cold arctype cathode extendingaxially along the center of said cylinder; a pair of metallicdisk-shaped cathode support members supporting said cathode and formingend walls for said device; a plurality of insulating ceramic membersinterposed between said end .wall members and said control electrodesupport members and between said control electrode support members andsaid anode member and hermetically sealed thereto to form anhermetically sealed envelope; and a highly purified xenon gas atmospherewithin said envelope, the product of gas pressure in millimeters and theradial spacing between anode and control electrode cylinders incentimeters being approximately equal to 2.5.

6. The device of claim 5 in which said metallic members are of titanium.

prising in stacked array a metallac cylindrical anode member; apair ofannular metallic grid support members in spaced relation with respectiveends of said anode member, the inside diameter of said grid supportmembers being substantially less than the inside diameter of said anodemember; a pair of metallic disk-shaped end wall members in spacedrelation with respective outer ends of said grid support members; aplurality of insulating ceramic spacers interposed between each pair ofadjacent metallic members and forming hermetic seals therewith to forman hermetically sealed envelope; a cylindrical metallic mesh gridelectrode extending between said grid support members and concentricwithin said anode member; an electron emissive cold arc-type cathodemember extending centrally along the axis of said cylindrical gridelectrode and between said end wall member said cathode exhibitingcharacteristics of low heat capacity and low thermal conductivity ascompared with metals and adapted to serve as the footpoint of an arcbetween anode and cathode in the absence of resistance heating thereof;and a highly purified noble gas atmosphere within said envelope.

8. The device of claim 7 in which said metallic members are of titanium.

9. A thyratron gaseous electric discharge device comprising a metallicanode cylinder member; a mesh control electrode cylinder concentric withsaid anode cylinder and interior thereof; a pair of metallic annularsupport members operatively connected with and supporting said controlelectrode; a cold arc-type cathode extending axially along the center ofsaid cylinders; a pair of metallic disc-shaped cathode support memberssupporting said cathode and forming end walls for said devices; a pairof insulating ceramic members interposed between said end wall membersand said control electrode support members; a second pair of insulatingceramic members interposed between said control electrode supportmembers and said anode member and hermetically sealed thereto to form anhermetically sealed envelope; and a highly purified gaseous atmosphereselected from the group consisting of xenon, helium, neon, argon andkrypton within said envelope, the product of gas pressure in millimetersof mercury and the radial spacing between said anode and said controlelectrode cylinders in centimeters being approximately equal to a numberhaving a value of 2.5 when the gaseous atmosphere utilized is xenon, 3.5when the gaseous atmosphere utilized is helium, 3.5 when the gaseousatmosphere utilized is neon,

1.0 when the gaseous atmosphere utilized is argon; and

2.25 when the gaseous atmosphere utilized is krypton.

10. A thyratron gaseous electric discharge device comprising a metallicanode cylinder member; a mesh control electrode cylinder concentric withand interior of said anode; a pair of metallic annular support membersoperatively connected with the ends of said control electrode; a coldarc-type cathode extending axially along the center of said cylindersand comprising a thermionically emissive active surface region adaptedto serve as the footpoint of an electric are between said anode and saidcathode in the absence of resistance heating thereof, said surfaceregion exhibiting the characteristics of low heat capacity and lowthermal conductivity as compared with metals; a pair of metallicdisc-shaped cathode support members supporting said cathode and formingend-walls for said device; a plurality of insulating ceramic membersinterposed between said end-wall members and said control electrodesupport members and between said control electrode support members andsaid anode member and hermetically sealed thereto to form anhermetically sealed envelope; and a highly purified stable noble gasatmosphere within said envelope.

11. A thyratron gaseous electric discharge device comprising a metallicanode cylinder member, a mesh control electrode cylinder concentric withsaid anode and interior thereof; a pair of metallic anular supportmembers'operatively connected with the ends of said control electrode; acold arc-type cathode extending axially along the center of saidcylinders; a pair of metallic disc-shaped cathode support memberssupporting said cathode andforming end-walls for said device; aplurality of insulating ceramic members interposed between said end-wallmembers and said control electrode support members and between saidcontrol electrode support members and said anode member and hermeticallysealed thereto to form an hermetically sealed envelope; and a highlypurified stable noble gas filling at a pressure of approximately l-3 mm.of mercury within said envelope.

12. A thyratron gaseous electric discharge device comprising a metallicanode cylinder member; a mesh control electrode cylinder concentric withsaid anode cylinder and interior thereof; a pair of metallic annularcontrol electrode support members operatively connected with the ends ofsaid control electrode; a cold arc-type cathode extending axially alongthe center of said cylinder and comprising a highly thermionicallyemissive surface region adapted to form the footpoint of an electric arebetween said cathode and said anode, said surface region exhibiting lowthermal conductivity and low heat capacity as compared with metals; apair of metallic disc-shaped cathode support members supporting saidcathode and forming end-walls for said device; a plurality of insulatingceramic members interposed between said end-wall mem- .anode members andhermetically sealed thereto to form an hermetically sealed envelope; anda highly purified stable noble gas atmosphere at a pressure ofapproximately l-3 mm. of mercury within said envelope.

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